The cement in most common use is Portland cement, which is made by finely grinding limestone and clay or shale and calcining with some added gypsum, to temperatures approaching or in excess of 1600 degrees centigrade. The mixture after calcining is known as clinker and requires further fine grinding, and frequently the addition of gypsum, to produce Portland cement. In some locations volcanic rocks can be substituted for the clay and shale.
Portland cement when mixed with water and aggregates sets to a concrete which, according to ASTM (Australian Standard) requirements, should achieve a compressive strength of 3000 psi or 20 mPa after 28 days. To achieve maximum mechanical strength, the amount of water used in the mixing must be kept to a minimum, and the casting of concrete made with Portland cement is consequently difficult if high mechanical strength is required. Surface treatment of cast concrete is also usually necessary to improve appearance.
Portland cement is presently produced in very large quantities, but, because of the need to finely grind the materials both before and after calcining, and because of the cost of achieving the high calcining temperatures, and the cost component for energy requirements, the cost is very high (by comparison with this invention). For example, in South Australia, the energy cost is about six times the cost of the basic materials.
An object of the invention is to provide a cement which can be produced without the need to finely grind the natural limestone and shale raw materials, or the calcined clinker, and without the need to attain high calcining temperatures, and yet be able to make use of readily available and inexpensive basic materials.
Various types of magnesium cement are already known, the most relevant to this invention being Sorel cement. In the production of Sorel cement, high grade magnesite or magnesium carbonate is calcined to form reactive magnesium oxide (MgO). If the calcining temperature is raised to 1500.degree. C. or higher, a non reactive product called deadburned magnesia is obtained. This product finds application in blast furnaces and in refractory applications. It lacks mechanical strength, however, and is not used where significant mechanical strength is required. It is not used in Sorel cement.
If on the other hand temperatures of calcining are reduced to not less than 750.degree. C., (and quite commonly 900.degree. C.) reactive or caustic magnesia is produced. This material has a useful mechanical strength, although when maintained in a moist condition for long periods of time, it very slowly converts to basic magnesium carbonate. This reaction is much slower than the corresponding reaction involving quick lime or hydrated lime, and many years are required to achieve a stable mechanical strength.
Caustic magnesium oxide produced by calcining magnesite at temperatures between 750.degree. C. and 1500.degree. C. will, however, react at ambient temperature with moderately concentrated solutions of magnesium chloride to produce Sorel cement. Sorel cement made in this manner is frequently erroneously referred to as magnesium oxychloride cement. Magnesium sulphate has been substituted for magnesium chloride to produce cements of lesser mechanical strength and severe shrinkage characteristics which have been erroneously referred to as magnesium oxysulphate cements.
The term magnesium oxychloride implies a formula Mg--O--Cl. Since these products produce hydrogen chloride on heating in a dry atmosphere, this formula is obviously incorrect. Consequently some writers have given the so-called magnesium oxychloride components the formula MgO--HCl.
This formula is completely at variance with chemical analysis of "sorel" cements, which have been given formulae as below by various authorities.
All reliable analyses of these "oxychloride" cements agree on some or all of the following empirical formulas being acceptable.
______________________________________ Ratio Mg Ratio Formula Mg:Cl Content Mg:OH ______________________________________ 2 Mg (OH).sub.2 Mg Cl.sub.2 4 H.sub.2 O 1.0 Mg = 25% 0.52 3 Mg (OH).sub.2 Mg Cl.sub.2 8 H.sub.2 O 1.35 Mg = 23% 0.40 5 Mg (OH).sub.2 Mg Cl.sub.2 8 H.sub.2 O 2.0 Mg = 27% 0.47 9 Mg (OH).sub.2 Mg Cl.sub.2 5 H.sub.2 O 3.38 Mg = 34% 0.61 ______________________________________
All these products result from an interaction of reactive magnesium oxide with magnesium chloride in aqueous solution, at temperatures between 0.degree. C. and 100.degree. C.
Numerous other authorities still accept the 2,3, 5 or 9 compounds as separate compounds with H.sub.2 O content varying significantly with the conditions of formation.
All sorel or magnesium oxychloride cements referred to in the literature comply approximately with the formulae given above and all decompose on heating to about 600 degrees C., evolving hydrochloric acid and leaving a residue of magnesium oxide. They are not resistant to continuous immersion in water, and, having a pH of between 4.8 and 5.2, are corrosive to steel.
Another commonly used cement is formed by calcining limestone minerals with high magnesium content, also at very high temperature, along with clays and shales, to produce a product similar to Portland cement.